Leaching of nickel laterites, or in in general of metal laterite ores, with sulphuric acid is a widely applied process. Normally, these processes are either carried out in tanks at elevated temperatures of 85-90° C. for periods of up to 22 hours or in pressure autoclaves under high pressures above 40 atm and temperatures around 250° C. In some cases, nickel containing materials are subjected to heap leaching for periods up to 18 months.
The article ‘Recovery of vanadium, potassium and iron from a spent catalyst using urea solution” by Mazurek K, Bialowicz K. Trypuć M.; Hydrometallurgy 103 (1-4), pp. 19-24, 2010’ describes a method for leaching of metals (vanadium, iron and potassium) using a mixture of sulphuric acid and urea.
U.S. Pat. No. 5,320,665 describes a process for recovery of, particularly, noble metals by leaching the ore with a solution of sulphuric acid and thiourea.
Document WO 200508548 describes a process wherein all types of metal including precious metals can be leached or dissolved from different metal containing materials such as soil and crushed rock by exposing the material to a combination of three different chemicals of a metal leachant which preferably comprises urea, a pH adjuster, preferably hydrolic acid, sulphuric acid or sodium sulphate, to lower the pH to a value below 3,5, and an oxidizer, preferably ferric chloride, at low temperatures.
Numerous patents and processes are available to explain leaching of metals with sulphuric acid. However, in most of these processes, leaching time is normally rather long for high metal recoveries leading to higher capital and operating expenses.